yeasts. However, some form of natural leaven is still used by many specialty bakeries. Sourdough starter is traditionally made with a small amount of old dough, preferably saved from a prior batch. This is traditionally called mother dough or chef, or in more modern usage, seed sour. First-generation starter or spontaneous seed may be created by storing new dough in a warm place and allowing sufficient time for it to sour. This small amount of old-dough starter contains the culture, and its weight is increased by additions of new dough and mixing or kneading followed by rest or leavening periods. A small amount of the resulting dough is then saved to use as starter sour for the next batch. As long as this starter culture is fed flour and water weekly, it can stay at room temperature indefinitely.

Sourdough bread is made by combining the increased amount of starter with another new-dough addition, along with any other desired ingredients to make the final dough. The starter comprises about 13 to 25% of the final dough, though particular formulas vary. This final dough may be divided and shaped, then allowed to rise, followed by baking.

It is not uncommon for a baker's starter dough to have years of history, from many hundreds of previous batches. As a result, each bakery's sourdough has a distinct taste. The combination of starter processes, refreshment ratios and rest times, culture and air temperature, humidity, and elevation also makes each batch of sourdough different.

A sourdough starter is a stable symbiotic culture of bacteria and yeast present in a mixture of flour and water. The yeasts Candida milleri or Saccharomyces exiguus usually populate sourdough cultures symbiotically with Lactobacillus sanfranciscensis.

The perfect yeast S. exiguus is related to the imperfect yeasts C. milleri and C. holmii; while Torulopsis holmii, Torula holmii, and S. rosei are synonyms used more frequently prior to 1978. C. milleri and C. holmii are physiologically similar, but DNA testing established them as distinct. Other yeasts reported found include C. humilis, C. krusei, Pichia anomaola, C. peliculosa, P. membranifaciens, and C. valida. There have been taxonomic changes in recent decades.

In general, San Francisco sourdough is the same as a Type I sourdough. Type I sourdoughs have a pH range of 3.8 to 4.5 and are fermented in a room-temperature range of 20 to 30 °C (68 to 86 °F); Saccharomyces exiguus leavens the dough, Lactobacillus sanfranciscensis and L. pontis highlight a lactic-acid bacterial flora that includes L. fermentum, L. fructivorans, L. brevis, and L. paralimentarius. In Type II sourdoughs Saccharomyces cerevisiae is added to leaven the dough, L. pontis and L. panis highlight the flora. These sourdoughs have a pH less than 3.5 and are fermented within a temperature range of 30 to 50 °C (86 to 122 °F) for several days without feedings, which reduces the flora's activity.

A traditional spontaneous starter will consist of basic items such as: water, bread flour, rye flour, and days of time. This mother dough is maintained with daily new-dough refreshments. In the lead-up to a batch of final dough, water and flour are traditionally added in time increments of portions of a day or hours. This is often a triple-iterative process, and one that not only has the effect of increasing the total amount of dough, but also may alter the yeasts:lactobacilli ratio. A time-period term encountered in the literature is accelerate, which is a reference to decreasing the time intervals between new-dough refreshments to increase the rate of gas production. Depending on the locale of the bakery and the type of bread being made, the starter can be either a relatively fluid batter or a stiffer dough. Firm starters (such as the Flemish Desem starter) are often more resource-intensive, traditionally being buried in a large container of flour to prevent drying out.

Faster, fewer-iteration starter processes have also been devised, sometimes using commercial sourdough starters as inoculants. The phrase starter culture is not well defined or standardized, and purchased starters in general fall into one of two subgroups. One product type is made from traditionally maintained and stable starter doughs, often is dried, but the precise microorganism ratios are unknown. The other starter subgroup is composed of cultured organisms carefully isolated from Petri dishes, grown into large, homogeneous populations in fermentors, and eventually processed into combined baker's products with numerically defined ratios and known quantities of microorganisms well suited to particular bread styles.

A fresh culture begins with a mixture of flour and water. Fresh flour naturally contains a wide variety of yeast and bacterial spores. When wheat flour contacts water, naturally occurring amylase enzymes break down the starch into the disaccharide maltose; maltase converts the sugar into glucose, which yeast can metabolize. L. sanfranciscensis prefers to consume maltose, while C. milleri is maltase negative. The mixture develops a balanced, symbiotic culture after repeated feedings.

Several methods are used to increase the chances of creating a stable culture. Unbleached, unbromated flour contains more microorganisms than more processed flours. Bran-containing (wholemeal) flour provides the greatest variety of organisms and additional minerals, though some cultures use an initial mixture of white flour and rye flour or "seed" the culture using unwashed organic grapes (for the wild yeasts on their skins). Grapes and grape must are also sources of lactic acid bacteria, as are many other edible plants. Basil leaves are soaked in room-temperature water for an hour to seed traditional Greek sourdough. Using water from boiled potatoes increases the leavening power of the bacteria, by providing additional starch. Some bakers recommend unchlorinated water for feeding cultures. Adding a small quantity of diastatic malt provides maltase and simple sugars to support the yeasts initially. The flour-water mixture can also be inoculated from a previously maintained culture. The culture is stable because of its ability to prevent colonization by other yeasts and bacteria as a result of its acidity and other antibacterial agents. As a result, many sourdough bread varieties tend to be relatively resistant to spoilage and mold.

Sourdough starter can be used in two different manners. Traditionally, a certain amount of sourdough starter (20 to 25 percent on average, depending on the water content of the starter) is mixed into the bread dough, and the bread is kneaded and allowed to rise as normal. The process is largely similar to using a pure strain of baker's yeast, although some care must be taken since the rise time of most sourdough starters is usually somewhat longer than the average for typical baker's yeasts. (As a result, many sourdough starters are unsuitable for use in a bread machine.) When using a particularly liquid starter with a high concentration of lactobacillus or acetic acid bacteria, the large amount of lactic and acetic acids produced needs to be managed carefully, since the acid can break down the gluten in the bread dough; this becomes less of a concern in a stiffer starter, where the yeast usually predominates.

The other manner of using sourdough starter is common for making quick breads or foods like pancakes. It involves using baking soda (and sometimes baking powder) to neutralize some or all of the acid in the starter, with the acid-base reaction generating carbon dioxide to provide lift to the dough or batter in a manner very similar to Irish soda bread. This technique is particularly common in kitchens where the starter is intentionally kept off-balance, with a substantially high acid level, and is particularly associated with areas such as Alaska.

History of sourdough

Sourdough likely originated in Ancient Egyptian times around 1500 BC and was likely the first form of leavening available to bakers. Sourdough remained the usual form of leavening down into the European Middle Ages until being replaced by barm from the beer brewing process, and then later purpose-cultured yeast.

Bread made from 100 percent rye flour, which is very popular in the northern half of Europe, is usually leavened with sourdough. Baker's yeast is not useful as a leavening agent for rye bread, as rye does not contain enough gluten. The structure of rye bread is based primarily on the starch in the flour, as well as other carbohydrates known as pentosans; however, rye amylase is active at substantially higher temperatures than wheat amylase, causing the structure of the bread to disintegrate as the starches are broken down during cooking. The lowered pH of a sourdough starter, therefore, inactivates the amylases when heat cannot, allowing the carbohydrates in the bread to gel and set properly. In the southern part of Europe, where baguette and even panettone were originally made with wheat flour and rye flour, sourdough has become less common in recent times; it has been replaced by the faster-growing baker's yeast, sometimes supplemented with longer fermentation rests to allow for some bacterial activity to build flavor.

Sourdough was the main bread made in Northern California during the California Gold Rush, and it remains a part of the culture of San Francisco today. The bread became so common that "sourdough" became a general nickname for the gold prospectors. The nickname remains in "Sourdough Sam", the mascot of the San Francisco 49ers. A 'Sourdough' is also a nickname used in the North (Yukon/Alaska) for someone having spent an entire winter north of the Arctic Circle and refers to their tradition of protecting their Sourdough during the coldest months by keeping it close to their body.

The sourdough tradition was carried into Alaska and the western Canadian territories during the Klondike Gold Rush. Conventional leavenings such as yeast and baking soda were much less reliable in the conditions faced by the prospectors. Experienced miners and other settlers frequently carried a pouch of starter either around their neck or on a belt; these were fiercely guarded to keep from freezing. However, freezing does not kill a sourdough starter; excessive heat does. Old hands came to be called "sourdoughs", a term that is still applied to any Alaskan old-timer.

San Francisco sourdough is the most famous sourdough bread made in the U.S. today. In contrast to sourdough production in other areas of the country, the San Francisco variety has remained in continuous production since 1849, with some bakeries (e.g., Boudin Bakery among others) able to trace their starters back to California's Gold Rush period. It is a white bread characterized by a pronounced sourness (not all varieties are as sour as San Francisco sourdough), so much so that the dominant strain of lactobacillus in sourdough starters was named Lactobacillus sanfranciscensis. Sourdough also became popular because of its ability to combine well with seafoods and soups such as cioppino, clam chowder, and chili.

Sourdough has not enjoyed the popularity it once had since bread became mass-produced. However, many restaurant chains, such as Cracker Barrel, keep it as a menu staple. Manufacturers make up for the lack of yeast and bacterial culture by introducing into their dough an artificially-made mix known as bread improver.

Aside from what might be called plain sourdough bread, there are a number of other breads that use similar starters and techniques. Amish Friendship Bread uses a sourdough starter that includes sugar and milk. However, it is further leavened with baking powder and baking soda, making it more of a quick bread. A real Amish sourdough starter for bread is fed with sugar and potato flakes every 3–5 days. The German Pumpernickel is traditionally made from a sourdough starter, although modern pumpernickel loaves often use commercial yeasts, sometimes spiked with citric acid or lactic acid to inactivate the amylases in the rye flour. Also, the Flemish Desem bread is a popular form of whole-wheat sourdough, though cultured in a much less liquid medium.

Sourdough is a dough containing a Lactobacillus culture, usually in symbiotic combination with yeasts. It is one of two principal means of biological leavening in bread baking, along with the use of cultivated forms of yeast (Saccharomyces). It is of particular importance in baking rye-based breads, where yeast does not produce comparable results. In comparison with yeast-based breads, it produces a distinctively tangy or sour taste, mainly because of the lactic acid produced by the lactobacilli; the actual medium, known as "starter" or levain, is in essence an ancestral form of pre-ferment. In English-speaking countries, where wheat-based breads predominate, sourdough is no longer the standard method for bread leavening. It was gradually replaced, first by the use of barm from beermaking, then, after the confirmation of germ theory by Louis Pasteur, by cultured